RFID or radio frequency identification technology has been used in a variety of commercial applications such as inventory tracking and highway toll tags. In general, a transceiver tag or transponder transmits stored data by backscattering varying amounts of an electromagnetic field generated by an RFID reader. The RFID tag may be a passive device that derives its electrical energy from the received electromagnetic field or may be an active device that incorporates its own power source. The backscattered energy is then read by the RFID reader and the data is extracted therefrom.
Several technical hurdles must be overcome in order to make RFID work. Typically, the backscattered energy from the RFID tag contains relatively low power and has a short range. There is also a tendency for the transmitted signal to leak into the received signal path in the reader, thus introducing noise. Neither the distance between the RFID tag and reader nor the phase relationship between the backscattered signal and the local oscillator in the reader is known. The RFID system must also function where the RFID tag has a non-zero rate of displacement and/or acceleration toward or away from the RFID reader. In toll road applications, for example, it is desirable to permit a RFID tag a speed of at least 100 mph.
Because the RFID reader's local oscillator frequency is identical to that of the carrier frequency, the receiver is a homodyne detector. In a homodyne receiver, more than a single detected channel is required to detect the backscattered signal's amplitude modulation envelope because signal nulls may be present depending on the signal phase relative to the phase of the local oscillator. These signal nulls have traditionally been overcome by using a second detector or mixer that is at a 90 degree phase shift from the first local oscillator. The output of the two mixers are usually combined in an image-reject configuration, or alternatively, by processing the signals in the digital domain. However, both solutions have proven to be undesirable.